Diversion Planning

[eyeinthesky]

I have had some discussions regarding the selection of an alternate airport or runway and would appreciate some information.

I am aware that it is OK sometimes to use 27L as an alternative to 27R at LL subject to certain conditions. However, someone else has insisted that we should use an alternate which is at least 50 miles away from our original destination on the grounds that it should be in a different weather system and is unlikely to be experiencing the same weather which would make us divert from our original destination. This obviously has a major impact on fuel load vs payload.

I have been unable to find a reference to this. Can anyone point me in the direction of a document which specifies this or is it an old wives tale?

I should mention we are talking about Public Transport in Light category piston aircraft.

[m&v]

The Dept Alternate issue has always been fuzzy,more so now with transport types doing single eng auto lands!!the original concernwas having some place to go if thhe WX was too Low to return,ergo the closest'open'field was chosen.In some cases this might be quite distant in the case of widespread fog....So the powers that be said in view of engine reliabiliy one could go quite far on one..Nowadays the A320 can 'plan' an Alternate within 340kms Different types varied distances etc...Flying light twins out of vancouver years ago we would select the closest 'open' airport(usually within 30 miles).


thats 340 nautical miles not kms,,,,,

[eyeinthesky]

Thanks for that.

I was specifically talking about at destination alternate. The departure alternate does indeed have to be within 1 hour's flight time with one engine out.

Anything else to add?

[CJ Driver]

Destination (and en-route) alternates

If you are public transport your Ops Manual will tell you the answer, probably in Part A under "Determination of Aerodrome Operating Minima" or equivalent section. If you are private, you should form your own policy.

In our case, to select as an alternate, we need the forecasts and actuals to indicate that for +/- 1 hour from expected arrival the weather will be at or above the minima for the second best approach. I've paraphrased that last bit 'cos there's a whole table of choices, but broadly if the alternate has got an ILS and a VOR non-precision approach, you have to plan for the non-precision minima, if it's got a straight-in or a circling, you have to plan for the circle, etc. In other words, you have to allow for being unlucky when you divert. Of course, when you get to your diversion field, you don't have to fly the NDB-circle-to-land just out of spite even though the ILS is working

So, to go back to your question, the alternate depends on actual likely weather, not on distance. Where the topography is simple, a widespread forecast mist or fog can force an alternate 100 miles or more from your original destination. Where the topography encourages local weather variations, your alternate might be just over a ridge line (or whatever) where the weather is quite different. Although I have also heard the comment about alternates being "50 miles away" there's nothing magic about the distance - it's only the weather that matters, and there is certainly no requirement in our ops manual on distance.

And yes, on a reasonable day, your alternate can be just a mile or two away (although I believe it should always be a different airport - your destination could be closed by an emergency, a power failure or a blocked runway, not weather).

[eyeinthesky]

Thanks.

The Ops Manual does indeed have all the criteria you mention, and I would have thought we should use that. What the company has tried to do is produce a 'generic' PLOG for the various destinations with an alternate which is 50 miles away. My argument has always been that, provided the alternate has the weather, it can be 5 miles away, but the CP has insisted on the 50 mile rule. In practice, were the weather to be below limits at original destination to be unsuitable we would go to the nearest suitable airfield rather than necessarily the planned distant diversion. We would of course not be able to launch without the weather being right at this alternate, but it seems daft to carry fuel for somewhere 50 miles away when 5 will do.

It all seems to be leading down the road I believe to be right: You can plan for an alternate generally which is 5 miles away or whatever, and for the 1% of time that that is unsuitable you carry extra fuel and plan for the distant alternate.

I may not have expressed this very well, but I hope you get my meaning.

[m&v]

Curios why the CP stipulates,and maintains,the 50mile 'rule'..Over here ,like the last commentor says.as long as the wx meets the Legal Dispatch criteria(600/2-800/2-or VFR depending on the 'forecast' one uses-term/area),one should be able to select the closest operational airport.Bit of a bind bussing Pax 50miles??

[Basil]

Under appropriate weather conditions it is usually permitted to commit to destination, one of the conditions being the availability of two runways.
I flew for a Persian Gulf outfit which interpreted this particular requirement as both ends of the same runway, e.g. 30/12 - wits!

[flyburg]

In our operation we have an automated flightplanning system(it checks route, wx, runways, notams, etc) and it will automatically select the least fuel alternate. The system does not look at commercial implications. Only if the weather becomes at or below minimums we have to call dispatch and look at other factors taking into account the commercial operation. On the distance, there is no minimum distance but there is a max distance. For "two engine operation" you always have to be within one hour(no wind) single engine flying time of an adequate alternate. Unless of course you are ETOPS approved.

Greetings, Mark